1. Field of the Invention
The invention relates generally to global positioning system (GPS) receivers and more particularly to a GPS receiver having a ratio-based signal acquisition method.
2. Description of the Prior Art
All global positioning system (GPS) receivers have some signal acquisition method for acquiring the GPS signal. Prior GPS receivers used threshold-based acquisition methods where signal acquisition is detected when the largest GPS correlation value exceeds a selected threshold. FIG. 1A is a chart showing three correlation value plots A, B, and C with respect to a fixed threshold versus code phase for three signal environments A, B and C, respectively. Signal environment A represents an open sky environment, signal environment B represents a case where ambient noise is high, and signal environment C represents a case where all noise and signals are attenuated.
Most prior art GPS receivers use the same fixed threshold for all signal environments. For the open sky environment A, a largest correlation value, signal A Vpk, exceeds the threshold at a code phase A Ppk. In this case, the prior art receiver successfully finds signal power at the code phase A Ppk. However in the high noise signal environment B, the noise results in a largest correlation value, noise B Vpk, at a code phase B Ppk. The correlation value B Vpk exceeds the threshold so the prior art threshold-based receiver falsely detects power at the code phase B Ppk. The false detection results in excess time and battery power being used. In the high attenuation environment C, both noise and signal are attenuated. The attenuation results in the lower correlation values. The largest correlation value, signal C Vpk, is greater than the noise in the signal environment C but, due to the attenuation, the correlation value C Vpk does not exceed the threshold. The prior art threshold-based receiver cannot detect the signal power at the code phase C Ppk and the true signal is missed.
There have been several attempts to maximize the probably of detecting true signal power and minimize the probably of false detections by varying the threshold according to the signal environment. Either a human user enters the environment type or the receiver attempts to determine the environment from characteristics of the incoming signal. However, none of these attempts have been entirely successful.
Therefore, there is a need for a signal acquisition algorithm that avoids false signal detection in a high noise signal environment and detects a true signal in a high attenuation signal environment.